This invention relates to an electrophotographic photosensitive member used for forming an image by utilizing electromagnetic waves such as ultraviolet rays, visible ray, infrared rays, X-rays, and the like. More specifically, the present invention relates to a photoreceptor used for electrophotography and having a photoconductive layer which comprises an amorphous material with, silicon as a constituent atom.
As a constituent for a photoreceptor, a solid-state image pickup device and so on, a photoconductive layer is required to have various characteristics such as a high and panchromatic sensitivity, a high S/N ratio and so forth, and to be harmless to humans. To cite one example, electrostatic charges are applied onto a photoconductive layer by corona discharge in electrophotography. Next, pairs of electrons and holes, i.e., photocarriers are generated so that the charges on the photoconductive layer are neutralized when the photoconductive layer is exposed to light. For example, positive charges on the photoconductive layer are neutralized by electrons generated in the photoconductive layer according to light exposure. Thus, an electrostatic latent image of a positive type is formed on the photoconductive layer. This latent image is visualized by depositing colored particles called toner on the photoconductive layer. The toner is oppositely charged to the charges on the photoconductive layer so as to be attracted by coulomb forces. Generally, a measure for heightening a potential of a toner depositing unit is introduced so that an electric field in reverse direction to that due to the latent image is established between the photoconductive layer and the toner depositing unit to avoid fog resulting from the inadequate triboelectric charges of toner. This measure is called developing bias. In this imaging method of electrophotography, the requirements for the photoreceptor are as follows:
(1) Charges produced by corona discharge may be retained until light exposure is performed; and
(2) photocarriers generated by light exposure may reach instantly to the surface of the photoreceptor so as to neutralize the charges on the photoreceptor without recombination. To meet these requirements, non-crystalline chalcogenide has been conventionally used for photoconductive materials to constitute a photoreceptor. This material has an excellent photoconductivity; however, the limit of light absorption lies in the ultraviolet wavelength region. As a result, its sensitivity to visible rays is not sufficient to form clear images. Moreover, the hardness of the material is too low to provide a long life for the photoreceptor.
From such a standpoint, amorphous silicon (hereinafter abbreviated as "a-Si") has attracted attention for use as photoconductive material. This a-Si represents a high and panchromatic sensitivity and a high hardness and is harmless to humans. On the other hand, its dark resistivity lies in the range from about 10.sup.8 ohm-cm to 10.sup.10 ohm-cm. (See: "Electronic properties of substitutionally doped amorphous Si and Ge", in PHILOSOPHICAL MAGAZINE 1976, Vol. 33, No. 6, pp. 935-949, by W. E. Spear et al). Therefore, it may not retain electrostatic charges thereon for a long time. In view of the above-mentioned problems, attempts have been made to use a multi-layered photoreceptor comprising a substrate, a barrier layer and an a-Si photoconductive layer in electrophotography. For the barrier layer of this multi-layered photoreceptor, an insulator layer such as silicon nitride and silicon oxide or an a-Si semiconductor layer of p-type and n-type may be introduced. The conductive type of the semiconductor layer depends on the charging polarity of the photoreceptor. Thus, a high charge acceptability and retentivity may be achieved according to this multi-layered structure. When the insulator layer is used for the barrier layer, however, a residual potential will be increased in proportion to the thickness thereof, because the insulator layer operates to block not only carriers injected from the substrate but also photocarriers generated in the photoconductive layer. On the other hand, breakdown due to the developing bias mentioned above may occur when the thickness of the insulator is established too thin in order to prevent it from blocking the generated photocarriers. In the case of the semiconductor layer, impurities may be contained to govern the conductive type thereof. Responding to the contents of the doped impurity, however, strain in the layer is increased. As a result, separation of layers may occur due to the difference in the strain in the respective layers.
Furthermore, all problems will not be completely solved by providing the barrier layer. For example, the photoconductive layer must have a high resistivity (i.e., a high dark resistivity) to retain charges thereon for a long time. To obtain a high charge acceptability, a covering layer needs to provide on the surface of the photoconductive layer. In many cases, this covering layer is made of an insulator having a high resistivity. The mobility of electrons in such a layer is very small. Therefore, the use of the covering insulator layer results in a low photosensitivity and a high residual potential.
Even with the a-Si photoreceptor having excellent photoconductive characteristics in the various points as described above, there still remains room to improve the characteristics or the structure of the a-Si photoreceptor.